A team of University of Colorado Boulder engineers has developed a scalable manufactured metamaterial, an engineered material with extraordinary properties not found in nature 鈥斕齮o act as a kind of air conditioning system for structures. It has the ability to cool objects even under direct sunlight with zero energy and water consumption.
When applied to a surface, the metamaterial film cools the object underneath by efficiently reflecting incoming solar energy back into space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation.
The new material, , could provide an eco-friendly means of supplementary cooling for thermoelectric power plants, which currently require large amounts of water and electricity to maintain the operating temperatures of their machinery.
The researchers鈥� glass-polymer hybrid material measures just 50 micrometers thick 鈥� slightly thicker than the aluminum foil found in a kitchen 鈥� and can be manufactured economically on rolls, making it a potentially viable large-scale technology for both residential and commercial applications.
鈥淲e feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology,鈥� said Xiaobo Yin, co-director of the research and an assistant professor who holds dual appointments in CU Boulder鈥檚 Department of Mechanical Engineering and the . Yin received DARPA鈥檚 Young Faculty Award in 2015.
The material takes advantage of passive radiative cooling, the process by which objects naturally shed heat in the form of infrared radiation, without consuming energy. Thermal radiation provides some natural nighttime cooling and is used for residential cooling in some areas, but daytime cooling has historically been more of a challenge. For a structure exposed to sunlight, even a small amount of directly-absorbed solar energy is enough to negate passive radiation.
The challenge for the CU Boulder researchers, then, was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath in order to achieve maximum spectral reflectance.
鈥淏oth the glass-polymer metamaterial formation and the silver coating are manufactured at scale on roll-to-roll processes,鈥� added Ronggui Yang, also a professor of mechanical engineering and a Fellow of the American Society of Mechanical Engineers.
听鈥淛ust 10 to 20 square meters of this material on the rooftop could nicely cool down a single-family house in summer,鈥� said Gang Tan, an associate professor in the University of Wyoming鈥檚 Department of Civil and Architectural Engineering and a co-author of the paper.听听
In addition to being useful for cooling of buildings and power plants, the material could also help improve the efficiency and lifetime of solar panels. In direct sunlight, panels can overheat to temperatures that hamper their ability to convert solar rays into electricity.
鈥淛ust by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency,鈥� said Yin. 鈥淭hat makes a big difference at scale.鈥�
The engineers have applied for a patent for the technology and are working with CU Boulder鈥檚 to explore potential commercial applications. They plan to create a 200-square-meter 鈥渃ooling farm鈥� prototype in Boulder in 2017.
The invention is the result of awarded in 2015 to Yang, Yin and Tang by the Energy Department鈥檚 .
鈥淭he key advantage of this technology is that it works 24/7 with no electricity or water usage,鈥� said Yang 鈥淲e鈥檙e excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more.鈥�
Co-authors of the new research include Yao Zhai, Yaoguang Ma and Dongliang Zhao of CU Boulder鈥檚 Department of Mechanical Engineering; Sabrina David of CU鈥檚 Materials Science and Engineering Program; and Runnan Lou of the .